Pressure-insensitive,sandwiched,acoustic material having a honeycomb core



3,504,761 PRESSURE-INSENSITIVE, SANDWICHED, ACOUSTIC MATERIAL April 7, 1970 s. F. SULLIVAN ET AL HAVING A HONEYGOMB CORE Filed June 18. 1969 INVENTORS. HARPER JOHN WHITEHOUSE SHELBY F. SULLIVAN BY ERVIN F. JOHNSTON ATTORNEY. JOHN STAN, AGENT.

United States Patent 3,504,761 PRESSURE-INSENSITIVE, SANDWICHED, ACOUS- TIC MATERIAL HAVING A HONEYCOMB CORE Shelby E.- Sullivan, Arcadia, and Harper John Whitehouse, Hacienda Heights, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed June 18, 1969, Ser. No. 834,439 Int. Cl. E04b 1/99 US. Cl. 181-33 6 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The purpose of this invention is to provide a composite, pressure-insensitive structure which may be used as an acoustic absorber underwater, for example with transducer arrays, to absorb undesirable acoustic reflections.

In the prior art, acoustic absorbers,.such as the B. F. Goodrich Co.s product called Soab, utilize the adsorption of an adhered gas film on the surface of aluminum flakes suspended in a matrix of butyl rubber. While acoustic absorption is good to depths of about 100 feet, hydrostatic pressure causes the adhered gas film to collapse, and the absorption of acoustic energy is pressure-sensitive and has a minimum value at depths below 200 feet.

In contrast, the acoustic material of this invention, due to its construction, is pressure-insensitive. The material consists of a sandwiched wall construction having a rigid honeycomb core, for example, of aluminum, capable of being subjected to a substantial compressive load. The inner facing at one end of the core is a noncompliant wall element, such as an aluminum plate of sufiicient thickness to be rigid. The outer facing, exposed to the acoustic energy, at the other end of the core is a compliant skin, such as stainless steel sheeting, a few mils thick. In addition, except where the acoustic material is used in a spherical configuration, lateral, rigid, and airtight walls are required.

Starting at the end adjoining the outer facing, the honeycomb cells are filled with a matrix of polyurethane containing a dispersion of aluminum flakes, for absorbing the acoustic energy. A film of gas adheres to the surface of each aluminum flake. The honeycomb cells are not filled to their full length, leaving a void in the end of the cells adjoining the inner, rigid, facing. A gas at ambient pressure is loaded into the voids at the inner facing end of the honeycomb cells.

An important feature of this invention is that the absorptive, acoustic, material is not subjected to hydrostatic pressure. Instead, the hydrostatic pressure is borne by the compliant skin in tension and the cellular support in compression, allowing the gas film absorber to be at ambient pressure equilibrium with the confined gas.

There are three diifercnt types of materials involved in the acoustic energy absorption: a hard material, the aluminum core structure; an intermediate absorbing material, the matrix of polyurethane and the aluminum flakes,

I Patented Apr. 7, 1970 ice with their adhered gas; and a soft material, the entrapped air.

The honeycomb core structure, which carries practically the whole load, being quite rigid, is not easily compressible. However, it does deform to some extent, causing a very slight compression of the entrapped air, more or less independently of the elastic compression of the honeycomb core. This is so because, for a given pressure, the mechanical modulus of the honeycomb core changes very slightly causing a correspondingly very slight change in the air pressure of the entrapped gas, and therefore an equally slight change in the volume of the gas.

While all the details about how the material herein described serves as an acoustic absorber are not known, the following details supply a plausible explanation of what probably transpires. When acoustical energy impinges upon the sandwiched acoustic material, it first produces a stress against the compliant skin, buckling it inwardly slightly, which causes a slight deformation of the absorber material, and therefore a slight compression of the entrapped gas.

It is believed that the acoustic energy absorption comes about from the frictional losses due to the sliding action of the aluminum flakes against the polyurethane, that is at the interface between the flakes and the polyurethane, thus causing a viscous loss. Apparently, the aluminum must be in a form having a large ratio of surface area to volume, inasmuch as aluminum spheres interspersed within a polyurethane binder in air do not work nearly as well. Moreover, if the polyurethane binder is mixed with the aluminum flakes in a vacuum, absorption of acoustical energy is greatly reduced, again indicating that the mechanism of energy absorption involves frictional losses involving the adhered layer of gas on the surface of the aluminum flakes.

The acoustic pressure on the polyurethane should not exceed roughly 25 to 50 p.s.i. If the pressure becomes excessive, the frictional action between the aluminum flakes and the polyurethane is inhibited and the absorption of acoustic energy does not take place. At any rate, the foregoing is believed to account for the action which takes place.

. Accordingly, an object of the present invention is to provide an absorbing acoustic material which is not subjected to hydrostatic pressure.

Another object is the provision of an acoustic material whose honeycomb structure gives it light weight and great structural rigidity in the direction of load-bearing.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a diagrammatic view, with two lateral side walls and part of the top compliant facing partially removed, of the pressure-insensitive acoustic material of this invention.

FIG. 2 is a cross-sectional view of the pressure-insensitive acoustic material in either a cylindrical or circular configuration.

Referring now to FIG. 1, therein is shown a pressureinsensitive, sandwiched, acoustic material '10 comprising a honeycomb core 12 of rigid material, for example, aluminum, having a direction of maximum support, or direction of maximum load bearing 13. The direction of maximum support :13, in the figure, coincides with the lines of intersection of the cell walls of the honeycomb structure. A compliant skin 14, which may be a sheet of stainless steel a few mils thick, is fastened at one end of the core 12 at an angle, usually a right angle, to the direction of maximum support 13 of the honeycomb core 12. Absorber material 16 partially fills the cells 18 of the honeycomb core 12. Air-mixed aluminum flake and polyurethane may constitute the absorber 16. A gas 20, for

example air at ambient pressure, fills the unfilled part of the honeycomb cells 18. A noncompliant rigid skin 22, which may be an aluminum plate, is fastened in an airtight manner to the other end of the honeycomb core 12, and, when planar, is parallel to the compliant skin .14. Where the sandwiched acoustic material has parallel sides, lateral sidewalls 24 would complete the structure 10. With respect to alternate embodiments, the compliant facing or skin 14 need not be of metal, but may be a flexi:

ble plastic or a fiber-reinforced resin. The facings 14 and 22 need not be planar as shown in FIG. 1, but may be of the acoustic material 10 need not be air, but may be nitrogen or some other gas or mixture of gases.

The absorber material '16 need not be a primarily singlematerial absorber but may be a composite mixture, such as a gas, or a mixture of gases, aluminum flake, encapsulated low-velocity liquid, and a resin matrix.

The absorber material v16 may be of a type such as polyurethane or rubber which adheres very strongly to the walls of the cellular core 12. In this case the compliant skin 14 need only be tacked onto the core 12 inasmuch as the absorber material 16 will not be shifted by the gas 20 in the cell pockets 18 below it, even if the gas has been compressed to some extent due to the depth of the water in which the sandwiched acoustic material 10 is located.

However, where the absorber material 16 is loosely packed in the top of the cells 18, the compliant skin 14 would have to be secured to the core structure 12 in an airtight manner.

The sandwiched, acoustic, material 10 of this invention can also be used for matching the acoustic impedance of a medium making contact with the compliant skin to that of the medium making contact with the non-compliant facing 22. To accomplish this, an acoustic matching section would be used between the noncompliant facing 22 and the absorber 16 in place of the gas 20, as disclosed in the same copending application.

The noncompliant facing 22 may be part of the mechanical structure upon which the acoustic material 10 is mounted; e.g., a deep-submergence pressure vehicle.

We claim:

1. A pressure-insensitive, sandwiched,'acoustic material comprising;

a honeycomb core of rigid material, defining at least one direction of maximum support;

a compliant skin secured at one end of the core at an angle to the direction of maximum support of the honeycomb core;

absorber material partially filling the cells of the honeycomb;

a noncompliant skin sealably secured to the other end of the honeycomb core and at the same angle to the direction of maximum support as the compliant skin; and

entrapped gas filling the unfilled part of the honeycomb cell.

2. The acoustic material according to claim 1, wherein the direction of maximum support is at right angles to the two skins.

3. The acoustic material according to claim 2, wherein the two skins are planar.

4. The acoustic material according to claim 2, wherein each of the skins are cylindrical and have a common axis.

5. The acoustic material according to claim 2, wherein each of the skins are spherical, the spheres having a common center.

6. The acoustic material according to claim 2, wherein the honeycomb core is aluminum;

the compliant skin is stainless steel sheeting;

the absorbermaterial is a matrix of air-mixed aluminum flake and. polyurethane;

the entrapped gas is air; and

the noncompliant skin is an aluminum plate.

References Cited FOREIGN PATENTS 723,621 2/1955 Great Britain.

ROBERT S. WARD, JR., Primary Examiner 

